I know, it is a (1:1) isolation transformer [wrong, see answer], found in some power supplies. However, when I search on this I get all kind of transformers except these. I have searched with the numbers on the components without any result.

Especially the middle one is very useful as ground loop isolator for analog audio (test results - great frequency response) and compact, however, got only one of these (two channels need two 'copies'). I know, there are audio transformers available and the reviews are mixed especially for the cheap ones, so I want to find a diversity of these transformers to be able to pick up two (at least) of these because I know it can be useful.

What is the exact name of these kind of transformers?

Two photos of the three I have:

transformers #1 transformers #2

update 18-march-2018:

The answer points out those are common mode filters/inductors/chokes. Played with many of these with great results. You can use these as a (1:1) isolation transformer however, you need a resistor in serie with the device for each channel on the input side to avoid pre-amp clipping on high gain/volume of the pre-amp on the input. These coil devices have a very low resistance, almost zero or just a few ohms. I figure out that it must at least 300 ohms, after adding a resistor of 300 ohms the clipping is gone.

  • \$\begingroup\$ Why not figure out what inductance you need and get a transformer that has it? \$\endgroup\$ Commented Mar 16, 2018 at 4:16
  • \$\begingroup\$ @IgnacioVazquez-Abrams: Thanks for the comment, however not the subject of the question. What's the exact name of this type of transformer, that's the question. \$\endgroup\$
    – Codebeat
    Commented Mar 16, 2018 at 4:22
  • 1
    \$\begingroup\$ Thanks for the update. If you put the resistor on the input of the "transformer" then you will have prevented overloading the preceding stage preamplifier output. That would explain the reduction in distortion. \$\endgroup\$
    – Transistor
    Commented Mar 18, 2018 at 21:49
  • \$\begingroup\$ @Transistor : Yep, that's actually the situation. \$\endgroup\$
    – Codebeat
    Commented Mar 18, 2018 at 22:01

2 Answers 2


Those are common mode filters/inductors/chokes. They suppress common mode noise created by switching power supplies, and are part of FCC requirements for noise filtering, along with X and Y series capacitors.

They have ferrite cores and generally only suppress high frequencies above audio range.

In more serious filters you would also have 2 single mode (2 wire) filters for noise not common to both hot and neutral, or two hot lines, either 120 or 180 degrees out of phase, depending on the power source.

Just to be clear, these are filters, not transformers.

A snippet of an article for more details:

Common Mode Choke

The impedance of an inductor is proportional to its inductance and the frequency of the signal passing through it. Inductors, therefore, can act like low-pass filters, that is they allow low frequency signals to pass through them, while they block higher frequency signals. A common mode choke is a special type of filter with two inductor windings on the same core. In order to understand common mode chokes, it is important to first understand what common mode means.

Normal Mode:

During normal operation of any circuit (such as a switch-mode power supply) connected to the AC Mains, AC current flows in through the AC Line and out through the AC neutral. This is called Normal Mode Current (or Differential Mode Current.)

Common Mode:

There are other noise currents generated in most circuits that tend to be higher in frequency. These currents can be conducted out the AC Mains in the form of noise. This is called Common Mode Noise. The current flows out on both the AC Line and AC neutral, hence the term Common Mode. It is this noise that a Common Mode Choke seeks to suppress.

How does a common mode choke work?

A common mode choke includes a magnetic core and two windings, making it a four-terminal device. The two windings have the same number of turns and are wound in opposite directions in a mirror configuration.

In this configuration, normal mode current flows into the circuit through one of the windings, and out through the other. The direction of current in each winding combined with the mirror winding configuration means the flux generated by each winding cancels out the flux generated by the other winding. Thus the common mode choke presents very low impedance to normal mode currents.

For common mode noise, however, the flux generated by the current in each winding is additive, causing the choke to present a very high impedance to common mode currents.

As stated above, the impedance of the common mode choke is proportional to the frequency of the current. So determining the required inductance of the choke depends on the lowest frequency of common mode noise the designer is trying to suppress.

The wire for a common mode choke must be larger enough in diameter to handle the Normal Mode current without overheating.

This is quoted from CET Technology.com

  • \$\begingroup\$ Great answer, that's exactly what it is, thanks. Take a look at it, thank you, found some already. \$\endgroup\$
    – Codebeat
    Commented Mar 16, 2018 at 4:33
  • 1
    \$\begingroup\$ I've handled a few of them in my time. \$\endgroup\$
    – user105652
    Commented Mar 16, 2018 at 4:34
  • \$\begingroup\$ Update: I figured out you need to use an resistor with these devices of at least 300 ohms to be usable as isolation transformers, see also my update at the bottom of my question. \$\endgroup\$
    – Codebeat
    Commented Mar 18, 2018 at 21:51

Although these Inductors are generally design to be balanced within ~2% for high CMRR with excellent low frequency response, they would work well as audio 1:1 isolation transformers. CM caps to gnd on either side improve CMRR and across windings for differential noise in both directions. Typically used on SMPS primary side.

www.token.com.tw/pdf/inductor.pdf See p57~63


  • \$\begingroup\$ Hi, thanks for the answer. I figured out you need to use an resistor with these devices of at least 300 ohms to be usable as isolation transformers, see also my update at the bottom of my question. \$\endgroup\$
    – Codebeat
    Commented Mar 18, 2018 at 21:50

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.